Here is a link to the book on Amazon.com. It's includes all the messages that Jesus gave directly to St. Faustina, a polish nun. Here are two excerpts from her diary:
March 15, 1937. Today, I entered into the bitterness of the Passion of the Lord Jesus. I suffered in a purely spiritual way. I learned how horrible sin was. God gave me to know the whole hideousness of sin. I learned in the depths of my soul how horrible sin was, even the smallest sin, and how much it tormented the soul of Jesus. I would rather suffer a thousand hells than commit even the smallest venial sin.
The Lord said to me, "I want to give myself to souls and to fill them with My love, but few there are who want to accept all the graces My love has intended for them. My grace is not lost; if the soul for whom it was intended does not accept it, another soul takes it."
This one speaks directly about Divine Mercy Sunday:
Our Lord also said, "I desire that priests proclaim this great mercy of Mine towards souls of sinners. Tell My priests that hardened sinners will repent on hearing their words when they speak about My unfathomable mercy, about the compassion I have for them in My Heart. To priests who proclaim and extol My mercy, I will give wondrous power; I will anoint their words and touch the hearts of those to whom they will speak." (Diary, 50, 1521) Easter Sunday is the best time to proclaim God’s mercy.
Here is a link about how to celebrate Divine Mercy Sunday, from which I took the last quote.
The chaplet of Divine Mercy can be said using any rosary. Here is a link that shows how to pray it.
There is so much more to be said about St. Faustina and Divine Mercy. Someday I will reread the Diary of St. Faustina and write some more posts on it. If you are not familiar with this devotion, I highly encourage you to learn more about it since there is a treasure trove of graces that come from this devotion.